1. Field of the Invention
The invention relates in general to a circuit apparatus, and more particularly to an amplifier circuit apparatus having compensation circuits for unequal input/output common mode voltages.
2. Description of the Related Art
The input and output common mode voltages Vcm of a cascaded amplifier circuit are usually designed to be equal. If the input/output Vcm does not match, an AC coupling circuit should be added to prevent operation failure of the amplifier due to bias point deviation. However, when power supply voltage decreases while the input/output Vcm remains the same, available output swing is thus reduced.
In a regular wireless system, an OP-type inverting amplifier is coupled next to a mixer, whereas the input/output Vcm of the amplifier and the mixer should be the same. Otherwise, a buffer should then be added in between the mixer and the inverting amplifier to serve as a level shifter.
Therefore, the trend of providing low-voltage supplies has raised the problem of how a cascaded amplifier circuit with unequal input/output Vcm should be designed.